Solid state sensor for carbon monoxide

ABSTRACT

A method of detecting a predetermined alarm condition in a combustion emission gas. The method comprises exposing to the gas a semiconductor gas sensor having a p-type mixed metal oxide semiconducting material of the first, second and/or third order transition metal series, the semiconducting material being responsive both to a change in concentration of a reducing gas in the surrounding atmosphere and to a change in concentration of oxygen in the surrounding atmosphere to exhibit a change in its electrical resistance. The resistance is monitored and an alarm signal is output if the resistance exceeds a predetermined value corresponding to the alarm condition.

The invention relates to a method and system for detecting apredetermined alarm condition in a combustion emission gas.

Concern over the generation of dangerous levels of CO by malfunctioningor incorrectly adjusted domestic gas appliances has been rising inrecent years. To comply with current ANSI standards in the US and everincreasing constraints on CO₂ emissions in the EU, there is anincreasing interest in combustion or flue monitoring technology.

Flue gas atmospheres represent particularly aggressive conditions.Temperatures range from 40° C. to 200° C. (depending on the degree ofcooling by the heat exchanger and whether or not the furnace is of anon-condensing or condensing design) while the gas itself is saturatedwith water vapour and creates reducing conditions due to low overalloxygen levels, typically ˜5%. Other components are CO₂ and CO, typicallyat around 8% and 30 ppm respectively, with the balance beingpredominantly nitrogen. In the event of the flue being restricted, or ofthe air/fuel pre-mix not being correct, the O₂ level in the fluedecreases. The CO level remains unchanged until the system becomesfuel-rich, whereupon it increases rapidly. FIG. 1 shows typicalcombustion behaviour for a pre-mixed boiler where such changes in O₂ andCO levels are clearly highlighted.

There are less commonly encountered situations in which significantchanges in CO or O₂ level may occur without a major change in theconcentration of the other species. Furthermore, “overgassing” may occurin which case fuel species or other partial combustion products such asH₂, CH₄ and heavier hydrocarbons can appear in the flue. A simple,reliable means of rapidly detecting either a fall in O₂ content or arise in CO level is therefore required, and if such means additionallyallows the detection of these other undesirable circumstances, this willconfer further advantage. Although the primary application addressedhere is that of a safety alarm activated in the event of malfunction, itwill also be clear that one or more of these conditions may also be usedto act as a control parameter to ensure safe and efficient operation ofthe combustion plant.

WO-A-93/08467 discloses a gas sensor for detecting more than one gas butthis requires separate sensing elements.

In accordance with a first aspect of the present invention, a method ofdetecting a predetermined alarm condition in a combustion emission gascomprises exposing to the gas a semiconductor gas sensor having a p-typesemiconducting material, the semiconducting material being responsiveboth to a change in concentration of a reducing gas in the surroundingatmosphere and to a change in concentration of oxygen in the surroundingatmosphere to exhibit a change in its electrical resistance; monitoringthe resistance; and outputting an alarm signal if the resistance exceedsa predetermined value corresponding to the alarm condition.

In accordance with a second aspect of the present invention, acombustion emission gas alarm system comprises a semiconductor gassensor having a p-type semiconducting material, the semiconductingmaterial being responsive both to a change in concentration of areducing gas in the surrounding atmosphere and to a change inconcentration of oxygen in the surrounding atmosphere to exhibit achange in its electrical resistance; and apparatus for monitoring theresistance of the semiconducting material and for issuing an alarmsignal if the resistance exceeds a predetermined value corresponding toan alarm condition.

Thus we use a semiconductor material which will sense oxygen, a reducinggas, or both in contrast to WO-A-93/08467 where separate sensors arerequired.

The invention preferably utilizes mixed metal oxides of the first,second and/or third order transition metal series. However, it isbelieved that metal oxides or even other materials may exhibit therequired properties.

Metal oxide semiconductor sensors typically operate at elevatedtemperatures somewhat higher than those encountered in small flues.Because of the demanding operating conditions, and their ability torespond to a number of parameters indicating potentially dangeroussituations, they represent a much more appropriate means of monitoringCO in this environment than other comparatively low cost sensors. Forexample, liquid electrolyte fuel cells are widely used in the industrialenvironment to detect dangerous levels of CO, but they are incapable ofsurviving for any extended period in the atmospheric conditions of theflue due to their reliance on aqueous electrolytes. Furthermore, theygenerally only respond significantly to a single chemical species, soseparate sensors would be required to measure CO and O₂. Catalyticsensors, on the other hand, lack sensitivity to CO at the toxic levelsof interest, are prone to poisoning of their catalysts and may giveambiguous or unreliable readings under changing oxygen levels.

Since the flue gas application is a safety critical one, where the livesof numerous persons adjacent to a malfunctioning boiler may be put injeopardy, sensing technologies which offer fail safe operation arenaturally preferred. The hot humid conditions within the flue, combinedwith the reducing nature of the flue gas and the potential occurrence ofpoisons requires that the chosen technique should be robust againstcorrosion and breakage of sensor connections, or loss in sensitivity dueto surface poisoning. Some semiconductor materials which are widely andsuccessfully used in other gas sensing applications are ill suited tothis demanding role. The most commonly employed types are based onn-type tin oxide additionally containing precious metal catalystadditives (for example those manufactured by Figaro and othercompanies), but these materials fail to meet the requirements of theapplication for a number of reasons;

(a) Although they can respond rapidly to changing oxygen levels asrequired, such responses may be wholly or partially irreversible due tobulk reduction of the oxide lattice. Such effects can occur even atcomparatively moderate operating temperatures.

(b) They have a limited ability to function in the presence of specieswhich can poison the surface sites governing the gas response. Moreover,such poisoning is not necessarily detectable other than by challengingthe device with a calibration gas mixture, which is an impracticalrequirement in a domestic situation.

(c) The increased resistance which they provide on contact failure is inopposition to the reduced resistance output which occurs on detection ofincreased levels of Co or reduced oxygen content. As such, it is notimmediately recognised by a simple signal processing system asindicative of a dangerous condition.

(d) They are particularly prone to interference from the effects ofwater vapour, which can swamp the signals derived from the species ofinterest.

In all these respects, the n-type tin oxide device does not fail safeand as such is unsuitable for the intended application.

Although much less widely used than n-type systems, p-type semiconductormaterials are known in gas sensing applications (see, for example,Chapter 4 in “Sensor Materials” by P. T. Moseley & A. J. Crocker, IoPPublishing 1996). However, their specific advantages in the demandingcombustion gas emission application have not previously been realised,appreciated or quantified.

We have found that p-type mixed metal oxide semiconducting sensors ofthe first, second and third order transition metal series areparticularly well suited for combustion gas emission, particularly fluegas, detection, for the following reasons;

(i) They exhibit excellent chemical stability in wet reducingatmospheres, due to the particularly high formation energies of theoxides.

(ii) They are resilient to the effects of typical poisons such asmercaptans and silicone sealants since they do not rely upon thepresence of precious metal catalysts to generate the gas sensitivesignal.

(iii) They undergo a rapid and reversible increase in resistance inresponse to a decrease in oxygen and/or an increase in reducing gas,e.g. CO, content of the surrounding atmosphere. The relationship betweenthe electrical resistance of such sensors, which is the responseparameter used, and the carbon monoxide and oxygen concentrations in thetest atmosphere follows a relationship of the form:R_(G)=A[O₂]^(−1/x)+B[O₂]^(−1/x)[CO]^(1/2)

where

-   -   R_(G) is the observed sensor resistance    -   [O₂] is the oxygen concentration    -   [CO] is the carbon monoxide concentration    -   A, B are constants which depend on the sensor resistance under        reference conditions    -   x is a parameter which depends on the point defect chemistry of        the oxide system. A typical value for x is 4.

There may be some departures to this relationship in cases where theflue temperatures are at the upper end of the range 40-200° C.,resulting in the volume percent of water in the atmosphere increasingdramatically. Notwithstanding this, the overriding importance of thisrelationship is that it means that each undesirable condition (increasedCO or decreased O₂) causes a change in resistance of the same sensewhich can be easily monitored.

(iv) They also possess a significant reversible response to otherreducing species of interest.

(v) Connection faults giving rise to an apparent resistance rise can beidentified as a dangerous state by a simple alarm system since thetarget gases will also produce a resistance increase.

Although a wide range of p-type materials are in principle suitable forsuch applications, the following examples are based on tests performedusing standard commercial devices marketed for CO monitoring (Capteursensor CAP07, City Technology Ltd). This design employs p-type oxides ofthe Cr—Ti—Mn—O system, for example as described in WO-A-01/88517,EP-A-0940673, EP-A-1135336 and EP-A-0656111. Other materials include CuOwith 10% TiO₂ and CoO with 5% TiO₂.

An example of a system and method for detecting a predetermined alarmcondition in a combustion emission gas will now be described withreference to the accompanying drawings, in which:

FIG. 1 illustrates typical combustion curves for a pre-mixed boiler;

FIG. 2 illustrates the variation in sensor resistance with carbonmonoxide concentration;

FIG. 3 illustrates the variation in sensor resistance with oxygenconcentration;

FIG. 4 illustrates the dependency of sensor resistance on carbonmonoxide and oxygen concentrations for a number of different sensors;

FIG. 5 illustrates the response of the sensor to a variety of differentgases in 5% oxygen and 24% relative humidity;

FIG. 6 illustrates the behaviour of three sensors in a flue atmospheretogether with an example of the response of an electrochemical CO sensorin a cooled extracted sample of the gas;

FIG. 7 is similar to FIG. 6 but in which a cooled extracted sample ofthe gas has been supplied to an electrochemical oxygen sensor;

FIG. 8 is a block diagram of the system;

FIG. 9 illustrates how resistance of the sensor is determined; and,

FIG. 10 illustrates the response of various p-type MMOS sensors in twocarbon monoxide/oxygen gases.

In this example, a combustion emission gas sensor is based on the use ofa p-type oxide of the Cr—Ti—O system. The use of such materials insensors is known and will be briefly described.

The sensor takes the form of a highly porous oxide layer, which isprinted down onto an alumina chip. The electrodes are co-planar andlocated at the oxide/chip interface. A heater track is present on thebackside of the chip to ensure the sensor runs “hot”. This is anecessary requirement as both the interference from humidity isminimized and the speed of response is increased. MMOS sensors do notnormally discriminate between different target gases. As such,considerable care is taken to ensure the microstructure of the oxide,its thickness and its running temperature are optimized to improveselectivity. In addition, selectivity is further enhanced through theuse of catalytic additives to the oxide, protective coatings and varioustypes of activated-carbon filters and on-chip catalytic oxide layers. Inthis example, the porous Cr—Ti—O oxide layer is coated with a catalyticoxide layer.

As can be seen in FIG. 8, the sensor 1 is connected to a heater driverbridge circuit 2 for controlling the sensor heater. An EEPROM (notshown) within the sensor 1 is connected to a microprocessor 3 while theoutput from the sensor 1 is connected to a simple amplification circuit4. The EEPROM contains heater control data corresponding to thecalibration temperature of the sensor. The circuit 2 and the circuit 4are powered from a power supply 5. The processor 3 generates an outputsignal which, in this case, is fed to an alarm which may be a visual oraudible alarm 6. In other cases, this signal could instead oradditionally be fed to a control system of a boiler or other equipmentgenerating the combustion emission gas which is being monitored.

As explained above, the sensor resistance increases with both increasingCO concentration (FIG. 2) and decreasing O₂ concentration (FIG. 3).Thus, whenever the air supply drops or when there is incompletecombustion for other reasons, the sensor will detect the condition as aresult of this combined effect. FIG. 4 demonstrates this for a range ofsensors exposed to various CO/O₂ combinations. It can be seen that thesensor resistance in 200 ppm CO at 10% O₂ is comparable to that in 100ppm CO at 5% O₂ which in turn is comparable to that at <50 ppm at 2.5%O₂. The sensors in this example were carefully selected from a standardbatch, representing the two extremes in performance, i.e. at both endsof the 95% confidence range.

In addition to the sensor 1 being alert to the presence of CO andvarying O₂ concentrations, it will also respond to the presence of otherrelevant gases, such as H₂, CH₄, and other heavier hydrocarbon fuels.FIG. 5 shows typical responses to these gases over. a range of differentconcentrations. The error bars represent the full range of responses for10 sensors. A continuous atmosphere of 5% O₂ was maintained in this testto replicate conditions in a boiler flue whose gas is being detected.

If the sensor 1 is to be considered for use in boiler flue applications,it is important that its performance should not degrade whilecontinuously operated over a time period commensurate with the life ofthe boiler or an acceptable maintenance interval. Longevity data is notas yet available for sensors operated within the flue. However, theperformance of similar devices operated under typical domesticconditions (for which application the sensor was originally designed)meets the 1 year test requirements of the UL2034 standard for domesticfire detection applications.

FIGS. 6 and 7 show the performance of 3 p-type sensors in the flue of acondensing gas furnace. The sensors are based on sensing layersemploying the Cr—Ti—O system and were set up to a resistance of 50 kohmsin clean air at 50% relative humidity. At this resistance, the sensorsare running at about 480-500° C. The sensors were installed in avertical tube ducting the flue gases away from the heat exchange coils.The temperature of the flue gases at this point was 40° C. As across-check, a sample of the flue gas was extracted and cooled and thendrawn across an electrochemical CO sensor (3F/F, City Technology Ltd)and an electrochemical O₂ sensor (2FO, City Technology Ltd). To createan unsafe condition, the vertical tube was restricted in stages by meansof a sliding plate. It can be seen from FIGS. 6 and 7 that the p-typesensors respond to both a reduction in oxygen level and an increase inCO level. The combined effects of these two responses gives rise to avery large signal from the p-type sensors which could readily be used inconjunction with a variety of simple signal processing means to alertusers of these potentially dangerous conditions.

It will be seen from the above discussion that alarm conditions causedby an increase in a toxic gas such as carbon monoxide and decrease inoxygen both cause an increase in resistance and this change inresistance is monitored by the processor 3 which will compare themonitored resistance with a predetermined threshold set such that if thethreshold is exceeded, this indicates a dangerous or alarm condition. Inthat situation, the alarm 6 is activated.

As explained above, the resistance rises proportional to the amount ofcarbon monoxide (or oxygen) present and a typical resistance range is 50KΩ (base line value with no gas present/clean air) to 150 KΩ. If thesensor is exposed to a sufficiently high current, polarization of thesensor material may occur. This requires that the sensor be measuredusing a low, <0.1V reference voltage. This is achieved by using a simplepotential divider as shown in FIG. 9.

A voltage reference is generated and appropriate resistors 11,12 chosento generate a voltage across the sensor of 0.1V or less. The sensingelement 13 of the sensor is connected in series with a 50 KΩ resistor 14with the 0.1V signal applied across both resistors. The output from thesensor is then taken from the point is between the two resistors. Thisoutput voltage is amplified to a sensible value, typically a gain ofabout 100, using the amplification circuit 4 to bring the signal withthe range of the analog-to-digital converter input of the microprocessor3.

The sensor described above uses a Cr—Ti—O material, for exampleCr—Ti—Mn—O. Other suitable materials include TiO₂ doped CoO and CuO.

EXAMPLE

Sensors from three different p-type gas-sensitive oxide systems, Co-O (JR Stetter, J Colloid Interface Science, 65 (1978) 432, and E MLogothetis et al, Appl Phys Letters, 26 (1975) 209), Cu—O (J Gentry andT A Jones, Sensors and Actuators, 4 (1983) 581-586) and Cr—Ti—Mn—O(EP-A-1135336) which display p-type behaviour were made up. For eachsystem, the Cr—Ti—O oxide layer in the standard City Technology COproduct, Cap07, was replaced with a layer comprised of one of itsoxides. For Co—O and Cu—O, TiO₂-doped compositions, CoO-5 wt % TiO2 andCuO-10 wt % TiO2 were used. Prior to being made into a screen-printableink, the oxide powders were either sieved through a 32 micron sieve(Cr—Ti—Mn—O) or a 125 micron sieve (Co—Ti—O, Cu—Ti—O). The specifictemperatures of sensor operation were 450° C. for both the Co—Ti—O andCu—Ti—O examples and 400° C. for the Cr—Ti—Mn—O example, respectively.

The sensors were initially exposed to air at 50% relative humidity (RH),followed by sn exposure to 1031 ppm CO in 21% O₂ for 15 minutes, aclean-up exposure in 50% RH air, an exposure to 1025 ppm CO in 1.5% O₂ ,and a final clean-up exposure in air at 50% RH. The results shown inFIG. 10 demonstrate that these materials respond to 1031 ppm CO but inaddition, the signal is further increased when exposed to a similar COlevel but with a greatly reduced O₂ level. It is therefore evident thatthese systems are sensitive to atmospheric conditions in which the COlevel increases and/or the O₂ level decreases.

1. A method of detecting a predetermined alarm condition in a combustionemission gas, the method comprising exposing to the gas a semiconductorgas sensor having a p-type semiconducting material, the semiconductingmaterial being responsive both to a change in concentration of areducing gas in the surrounding atmosphere and to a change inconcentration of oxygen in the surrounding atmosphere to exhibit achange in its electrical resistance; monitoring the resistance; andoutputting an alarm signal if the resistance exceeds a predeterminedvalue corresponding to the alarm condition.
 2. A method according toclaim 1, wherein the reducing gas is one of CO, H₂, CH₄ and higherhydrocarbons.
 3. A method according to claim 1 or claim 2, wherein theelectrical resistance of the semiconductor gas sensor is related to theconcentrations of oxygen and carbon monoxide in the surroundingatmosphere over at least a range of atmospheric compositions via anexpression of the form: R_(G)=A[O₂]^(−1/x)+B[O₂]^(−1/x) [CO]^(1/2) whereR_(G) is the observed sensor resistance [O₂] is the oxygen concentration[CO] is the carbon monoxide concentration A, B are constants whichdepend on the sensor resistance under reference conditions x is aparameter which depends on the point defect chemistry of the oxidesystem.
 4. A method according to any of the preceding claims, whereinthe p-type material comprises a metal oxide.
 5. A method according toany of claims 1 to 3, wherein the p-type material comprises a mixedmetal oxide.
 6. A method according to claim 4 or claim S, wherein themetal is of the first, second and/or third order transition metalseries.
 7. A method according to claim 6, wherein the semiconductormaterial comprises a p-type oxide of the Cr—Ti—O system.
 8. A methodaccording to claim 6, wherein the semiconductor material comprises ap-type Cr—Ti—Mn—O system, CuO with TiO₂ or CoO with TiO₂.
 9. A methodaccording to any of the preceding claims, wherein the combustionemission gas is a flue gas.
 10. A combustion emission gas alarm systemcomprising a semiconductor gas sensor having a p-type semiconductingmaterial, the semiconducting material being responsive both to a changein concentration of a reducing gas in the surrounding atmosphere and toa change in concentration of oxygen in the surrounding atmosphere toexhibit a change in its electrical resistance; and apparatus formonitoring the resistance of the semiconducting material and for issuingan alarm signal if the resistance exceeds a predetermined valuecorresponding to an alarm condition.
 11. A system according to claim 10,wherein the electrical resistance of the semiconductor gas sensor isrelated to the concentrations of oxygen and carbon monoxide in thesurrounding atmosphere over at least a range of atmospheric compositionsvia an expression of the form:R_(G)=A[O₂]^(−1/x)+B [O₂]^(−1/x [CO]) ^(1/2) where R_(G) is the observedsensor resistance [O₂] is the oxygen concentration [CO] is the carbonmonoxide concentration A, B are constants which depend on the sensorresistance under reference conditions x is a parameter which depends onthe point defect chemistry of the oxide system.
 12. A system accordingto claim 10 or claim 11, wherein the p-type material comprises a metaloxide.
 13. A system according to claim 10 or claim 11, wherein thep-type material comprises a mixed metal oxide.
 14. A system according toclaim 12 or claim 13, wherein the metal is of the first, second and/orthird order transition metal series.
 15. A system according to claim 13or claim 14, wherein the semiconductor material comprises a p-type oxideof the Cr—Ti—O system.
 16. A system according to claim 13 or claim 14,wherein the semiconductor material comprises a p-type CuO with TiO₂ orCoO with TiO₂.
 17. A system according to any of claims 10 to 16 mountedto or adjacent to a flue gas outlet so as to expose the sensor to a gasflue.